In recent years and continuing, integrated optical devices using silicon (Si) substrates are attracting attention. Optical devices using silicon substrates are fabricated at a low cost and monolithically mounted together with electronic circuits. Silicon is a medium transparent to an optical signal at 1.3 μm band or 1.55 μm band, which bands have been conventionally used in optical communication systems. Making use of semiconductor processes, low-loss and high-confinement silicon photonic waveguides can be fabricated. On the basis of such silicon photonic waveguides, a variety of optical devices are proposed and demonstrated.
In order to increase transmission capacity of silicon photonic integrated circuits, a wavelength-division multiplexing (WDM) based silicon photonic integrated circuit is a promising approach. WDM-based silicon photonic integrated circuits adopt a WDM transmission scheme used in fiber optic transmissions, and independently modulated optical signals with different wavelengths are multiplexed in a silicon device for transmission.
FIG. 1 illustrates a typical WDM-based optical transceiver 100. A transmission-side integrated optical circuit 110 and a receiving-side integrated optical circuit 120 are connected to each other with an N-fiber optic array 131. A 4-wavelength laser array 111, which serves as a WDM signal light source, is flip-chip mounted on a silicon substrate 113 in the transmission-side integrated optical circuit 110. Continuous-wave (CW) signal lights output from the respective channels of the 4-wavelength laser array ill are coupled to corresponding silicon photonic waveguides and guided to 1×N photo couplers 112-1 through 112-N. The signal light of each wavelength is equally split into N branches by the corresponding 1×N photo coupler 112. The light components split from the respective wavelengths are lined up in order of wavelength by branch-connection silicon waveguides 116, and modulated at an optical modulator array 117 under application of different data items depending on the wavelengths. The modulated signal lights are combined by an optical wavelength multiplexer (MUX) 113 into a single silicon photonic waveguide and input to an optical fiber 119 for transmission.
N sets of the above-described wavelength multiplexing mechanism are provided to the transmission-side integrated optical circuit 110 according to the number N of the divided branches. Each of the wavelength multiplexing mechanisms is optically connected, to a corresponding one of the optical fibers 119. The N-branch (or N-channel) signal lights are transmitted in parallel by N fiber optic array 131.
In the receiving-side integrated optical circuit 120, the WDM-based signal lights transmitted through the optical fibers 119-1 to 119-N are input to demultiplexers (DEMUX) 121-1 to 121-N, and separated into wavelength components. The separated wavelength components of each channel are output to different silicon photonic waveguides and converted into electric signals representing the transmitted data signals by an optical to electrical (O/E) converter that includes a photodetector array 123. N sets of the above-described separation/demodulation mechanism are provided in parallel in the receiving-side integrated optical circuit 120.
The total transmission capacity of the optical transceiver 100 is defined as D×N×M (Gb/s), wherein D denotes a modulation rate (Gb/s) of a modulators, N denotes the number of branches, and M denotes the number of wavelengths. In order to achieve a desired level of the total transmission capacity T (Gbs) using multiple WDM links, the number k of the laser arrays 111 (where k=T(N×M)) may be increased; however, the cost and the size also increase. If the number of laser arrays 111 is kept small, it is desired to increase the number N of the branches to supply as many signal lights as possible to the transmission paths from a single laser source. The minimum, sensitivity of a photo detector to the received light and the transmission loss of each link due to insertion of the optical modulator array 117, MUX 118 and DEMUX 121 are fixed in advance. Accordingly, in order to increase the number N of the branches for error-free transmission, the output level of the laser has to be increased to N times greater. In reality, the output power of currently available semiconductor laser sources is 10 mW to 100 mW and there is a limit to increasing the number of branches from the viewpoint of the laser output.
It is desired to provide an optical transceiver and an optical output level control technique that can reduce power consumption of the entirety of an optical transmission system. Meanwhile, a configuration of inserting a wideband semiconductor optical amplifier (SOA) before the demulplexing part in a receiving-side optical device is known. See, for example, Radhakrishnan Nagarajan, et al. “InP Photonic Integrated Circuits”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 16, No. 5, September/October.